I have Photo Stream set up and my pictures show up in iPhoto on my Mac. Do I have to import them to keep them permanently? How can I free up iCloud space?
Open the iCloud preference pane on your Mac to get the easiest way I've found to manage the storage.
Click the Manage... button in the lower right.
This will calculate all the storage users counting against your quota and offer you details on how to clean up each category. Once you've reviewed each bin - you can decide to delete or if you would rather purchase more storage to keep what you have stored in the cloud.
My experience is people can do one step to free up significant backup space from iOS. Go to the Settings app and then iCloud > Storage & Backup > Manage Storage
Select the device you are on and then under BACKUP OPTIONS - turn off Camera Roll and possibly any other items you don't want backed up.
This means you need another way to back up your photos, but since you have a Mac - you could set iPhoto to automatically back up all photo stream contents or regularly sync the photos and videos from iOS to the Mac and then back up the Mac to a cheaper hard drive or service than iCloud.
"My Photo Stream" does not consume space in your iCloud account storage quota so deleting photos from Photo Stream will not free up iCloud storage space.
If Photos.app has "My Photo Stream" turned on in the iCloud preferences, any photos that any device adds to "My Photo Stream" will get automatically imported into the Photos.app library on your Mac. They do not have to be manually imported.
In recent (2017) versions of Photos.app, individual images can be selected and deleted from "My Photo Stream", but be careful when deleting things. Photos deleted from "My Photo Stream" MAY also be removed from the local library on the device (depending on exactly how they were deleted: delete key; menu items; contextual menu; etc.), but they will not be removed from any other devices that have imported the images into their local library (which happens automatically with Photos.app on the Mac). Photos that were removed from the local library can be recovered from the "Recently Deleted" folder on the device, and are not added back to "My Photo Stream". Note that the photos visible in "My Photo Stream" can differ between different devices, perhaps due to time lag between local caches and server updates.
To reclaim iCloud storage space, remove data from other services such as Documents and Data, Mail, and iCloud backup. In particular, iOS device backups can consume a lot of iCloud space - you might consider having iOS devices backup to your Mac through iTunes rather than backing up to iCloud. This change can be made using iTunes when the iOS device is attached via a USB cable.